Array positioning system

ABSTRACT

A solar array positioning system includes first light sensor means facing the same direction as the solar array and second light sensor means facing the opposite direction. A third sensor, this one responsive to gravity, is employed to indicate the direction about a diurnal slew axis that the array is tilted. These sensors are employed to determine the initial array slewing direction to insure that during slewing and subsequent tracking, the array never is rotated through more than a given angle, less than 360°, between two fixed limits. This permits connection to the array by hard wiring rather than slip rings. The system includes also fourth sensor means for providing fine tracking signals which assume control of the array position during tracking. A vane structure reduces the sensitivity of various of the sensors to obscuration by foreign particles. A low cost uncorrected lens may be used with the fourth.

This is a division, of application Ser. No. 113,506, filed 1/21/1980,now U.S. Pat. No. 4,314,546.

The present invention relates to solar tracking systems, and moreparticularly, to solar sensor systems that generate electrical signalsfor operation of servo-motors for controlling the attitude of thetracking system.

Solar energy conversion system translate solar radiant energy into auseable form such as thermal energy or electricity. The transducingmeans (i.e. photovoltaic cells in the case of an electrical system) maybe relatively expensive and a significant part of the system cost. Oneway to reduce the cost of the system in the sense of obtaining greaterefficiency in the utilization of the solar energy, is to concentrate thesun's radiation onto the transducing means. Again using photovoltaiccells as an example, their active area in a system employing solarconcentrators, such as an array of lens, mirrors or the like, eachfocusing the same rays onto a different photovoltaic cell, may be lessthan 1% of that required for a system without a concentrator.

In most conversion systems which employ concentrators, a tracking systemis emmployed to control the attitude of the concentrators as a functionof the diurnal rotation and seasonal changes in orientation of the earthrelative to the sun, to maximize the radiation incident on thetransducer means. In such a tracking system, an error of only one halfdegree of arc represents a mis-aiming of more than one solar diameterwhich in many systems in unacceptable. During evening hours or cloudyweather when solar energy is not available, the concentrators (i.e. anarray of lenses) and their associated photovoltaic cells becomemisoriented with respect to the sun. When the sun again appears, thetracking system must be able to reorient the concentrator-photovoltaiccell array, hereafter simply termed the "CP array," over a wide angularrange relative to the polar axis, sometimes termed the diurnal axis, asdiscussed in more detail below.

Practical power output levels from a conversion system employing solarconcentrators can be obtained during the time the sun is at greater thansome minimal elevation, such as 10 degrees, above the horizon. Duringthe summer months at moderate latitudes, e.g., up to 58%, this can befor 7 1/6 hours before and 7 1/6 hours after local noon, a total of147/8 hours. Since each hour of operation requires a rotation of 15degrees about the polar axis of the concentrator, it is desirable thatthe CP array be capable of motion about this axis of at least ±1071/2degrees relative to its position at local noon. When the sun firstappears, the CP array can be in any orientation within this range. It ispossible, for example, that the concentrators of the CP array may befacing 180° away from the sun. In this case the tracking systeminitially will have to swing the CP array through an angle of at least180° to its correct orientation. If the tracking system were to alwaysrotate or slew the CP array in the same direction, then slip rings wouldbe required to provide electrical connection from the photovoltaic cellsto parts of the system which are not rotated. Slip rings are notconsidered to be sufficiently reliable for this application. Anotherproblem encountered in systems of the type discussed above is that onsunny days, bright clouds near the sun may cause signals to be producedby the tracking system sensor(s) which cause mis-aiming of the CP array.Another is that if these sensor(s) should become partially obscured byinsects, leaves, bird droppings, or other foreign matter, the trackingsystem may deliberately mis-orient the CP array in an attempt toovercome the resulting signal imbalance.

Aside from the problems discussed above, to be practical the trackingsystem should be inexpensive. This precludes certain designs thatotherwise might be attractive. For example, a clock driven system couldnot be allowed to slip without resulting in mis-aiming. Such a non-slipsystem would require a system rigid enough to withstand the anticipatedwind loading. Such rigidity implies a bulky and expensive drivemechanism which makes the system very costly.

Prior art tracking systems may use one of two different types ofsensors. One is known as an "integrating" sensor, in which someconversion device, such as a photodiode or photoconductive element, isilluminated by light collected over a wide range of solid angles. Thiscollection over a wide range of angles is made possible by diffusing thelight before it is incident on the conversion device. In one example, atranslucent hemisphere is placed over photodiodes situated on oppositesides of a tracking system. When the hemispheres are illuminatedunevenly so that a larger portion of one hemisphere is illuminated ascompared to the other, the current output of its photodiode has acorrespondingly higher value than that of the solar photodiode in theother hemisphere. This unbalance in output currents operates aservo-motor that drives the system in a direction to collect theimbalance. When the two outputs are equal, the hemispheres areilluminated symmetrically and the system is aimed directly at the sun.

Tracking the seasonal variation of the sun's apparent position requirestipping the system about an axis normal to the polar axis. An additionalpair of hemispherical sensors may be provided along the seasonal axisfor controlling the tracking system's position about that axis.

Integrating sensors have the advantage that they are responsive toillumination over a very large range of solid angles. Their sensitivity,i.e., the degree of electrical unbalance that is generated by a givendeparture of the sun's illumination direction from a symmetricalconfiguration, can be enhanced by the use of shadow vanes, which areopaque. The use of shadow vanes and how they enhance sensitivity isillustrated with more particularity in U.S. Pat. No. 4,151,408. Suchvanes, by increasing the sensitivity of the system, provide greateraiming accuracy.

A drawback of the systems described above is that equal outputs from thephotodiodes can also result when the sun is directly behind the trackingsystem. That is, the system may also track in a stable mode 180° out ofthe desired orientation.

The second kind of sensor which is termed an "imaging" sensor includeslenses or concave reflectors to form an image of the sun on the surfaceof the detector. The detector may be split into four 90° quadrants asshown, by way of example, in U.S. Pat. No. 4,041,307. Imaging sensorshave the advantage of high sensitivity to small angular misaiming. Animage at the focus of a well corrected lens (or reflector) also has theproperty that should the lens be partially obscured, the position of theimage does not change. Only the brightness of the image changes. Such asensor, therefore, should be nearly completely immune from mis-aimingdue to partial obscuration of the lens. An application for such a lensrequires that the lens be reasonably large in diameter, greater than 1",and a low f number to minimize partial obscuration and to achieve anappropriate angular field of view. However, taking costs intoconsideration, such a lens or reflector, if well corrected, isrelatively costly and is not desirable in an inexpensive trackingsystem.

A tracking system embodying the present invention comprises a pluralityof sensor means for producing control signals indicative of the initialorientation of a CP array. These include light sensors which indicatethe general direction to the sun and an attitude sensor for indicatingthe direction in which the CP array is tilted about the diurnal axis.Signals derived from these sensors are employed to slew the CP arraytoward its tracking position in a direction to insure that the arraynever rotates through more than a given angle (less than 360°) betweenfixed limits.

Additional features of a solar tracking system embodying the presentinvention include a light sensor (which may be employed for producingtracking signals) having an uncorrected lens at a given focal length andwherein solar sensor means are at a plane other than at the focal lengthto provide increased immunity from partial obscuration of the lens. Afurther feature of a system embodying the present invention includessolar sensor means and control means arranged to track the sun in adiurnal cycle. The sensor means including spaced voltaic generator meansand light diffusing means over the generator means including vane meansadjacent each generator means oriented to reduce the sensitivity of thegenerator means to the shadowing effect of partial obscuration of thelight diffusing means for foreign matter.

In the drawing

FIG. 1 is an isometric view of a tracking system embodying the presentinvention,

FIG. 2 is a side elevation view of the embodiment of FIG. 1 taken alonglines 2--2,

FIG. 3 is a bottom plan view of the embodiment of FIG. 1 taken alonglines 3--3 in FIG. 2,

FIG. 4 is a sectional view through a gravity attitude sensor switch,

FIG. 5 is a sectional view through an integrating sensor employed in theembodiment of FIG. 1,

FIG. 6 is a sectional elevational view through the imaging sensor of theembodiment of FIG. 1,

FIG. 7 is a schematic diagram illustrating the summing arrangements forsome of the photoelectric solar sensors employed in the embodiment ofFIG. 1,

FIG. 8 is a block diagram of the control system,

FIG. 9 is a diagram useful in explaining the principles of the presentinvention, and

FIGS. 10-14 are graphs useful in explaining the principles and operationof the tracking system of the present invention.

In FIG. 1 solar generator system 10 includes two identical sets of CParrays 12 and 14. The front of the arrays at 16 is normally aimed at thesun. Such arrays are well known and need not be described in detail.Generally, each array includes a transparent front cover at 16 which maybe glass, an array of concentrating lenses, and a corresponding array ofphotovoltaic cells for receiving concentrated solar energy from therespective lenses. In another form of array system, thermal generatorsmay be substituted for the photovoltaic generators. In general, forpurposes of the present invention, any kind of thermal conversion orelectrical conversion system may be employed with the tracking system tobe described.

The arrays 12 and 14 are pivotally mounted to yoke 18. The arrays 12 and14 pivot about the seasonal axis a_(s) in direction 20. The arrays 12and 14 are rotated in directions 20 by the seasonal drive means 22mounted to the yoke 18. The drive means 22 may be a servo motor andassociated gearing arrangement or in the alternative, could be any otherdrive system, such as a belt and pulley arrangement. The rotation of thearrays 12 and 14 about the seasonal axis a_(s) takes into considerationthe variations of the orientation of the earth's polar axis with respectto the sun during the annual period. These variations are 35 231/2° fromthe position that occurs at the equinox. During tracking, the axis a_(s)is always maintained as close to normal to the sun's rays as possible.When the axis a_(s) is normal to the sun's rays, it is horizontal at oneparticular time of day, local noon. The sun's orientation and the CParray's orientation at this time are reference orientations for thecontrol system to be described later. The term "reference orientation"will be used to describe these orientations. In this orientation, thenormal to the CP array will be considered to be at 0° and the line tothe sun also to be at 0°. The control system, to be described later,orients the CP array with respect to this reference orientation.

The yoke 18 is secured to support 24. Support 24 may be an elongatedpost whose long axis is perpendicular to the axis a_(s). Support 24 ispositioned centrally between the two arrays 12 and 14 and rotates aboutthe diurnal axis a_(d) which is parallel to the earth's polar axis. Thedirection of this axis remains fixed with respect to the earth. Support24 is rotatably mounted between two bearing supports 28 and 30 which aresecured to earth by legs 36, 38 and 40 in a tripod arrangement. Diurnaldrive means 32 is mounted to the bearing support 30. This drive meansmay include a servo motor and associated gear box for rotating thesupport 24 about the diurnal axis a_(d) in the directions 34. Clockwisedirections about axes a_(s) and a_(d) are indicated as positive whilethe counter clockwise directions are indicated as negative.

Solar radiation sensor S2 is secured to and coextensive with side 42 ofthe array 12 frame. Sensor S2 is mounted to a bracket 44, FIG. 2, whoseouter surface is coplanar with the plane outer surface of side 42.Sensor S2 is located below the rear surface 45 of the array 12. SensorS2 faces outwardly from the array in a direction parallel to axis a_(s).Sensor S1 is identical to sensor S2, is mounted in identical fashion toside 46 of array 14 directly opposite sensor S2 and facing in theopposite direction parallel to axis a_(s). Sensors S1 and S2 arecoplanar with the respective sides 46 and 42 of the arrays 14 and 12 andsituated below the rear surface 45 of the respective housings of thearrays so that the sides 42 and 46 serve also as shadow vanes for thesesensors.

Sensor S3 is identical to sensors S1 and S2 and is secured to a bracket48 which is connected to bracket 44 and extends therefrom. Bracket 48forms a base surface on which the sensor S3 is mounted. Sensor S3 facesin a direction normal to the plane in which the axes a_(d) and a_(s) lieand in a direction opposite to the direction in which the solar cells ofthe arrays 12 and 14 are facing. Thus, when the solar cells on arrays 12and 14 are aimed directly at the sun, the sensor S3 is underneath thearrays and in shadow.

A typical sensor S2 is shown in FIG. 5. The sensor S2 comprises twoidentical photodiodes 50 and 52 symmetrically mounted within atranslucent thermoplastic hemisphere 54 mounted on bracket 44. Thehemisphere 54 is slightly flattened at its peak to provide optimumdiffusion of light to the photodiodes. The hemisphere 54 is split intotwo semi-hemispheres or quarter spheres by opaque vane 56. Vane 56 is arectangular member which has an edge 58 coplanar with the face of thearray 12 (FIG. 1) and an opposite edge mounted to the bracket 44. Thevane 56 is a plane member whose plane is perpendicular to the directionof axis a_(d). This construction is generally typical of the remainingsensors S1 and S3. The exception with respect to sensor S3 is that itsvane 60 is mounted to the underside of bracket 48. Vane 60 extends in adirection perpendicular to the plane in which axes a_(d) and a_(s) lieand above the hemisphere 54 of sensor S3. The vanes 56 and 60 preferablylie in the same plane.

Thus, the photodiode 52, FIG. 5, of sensor S2 lies in one hemispherequadrant while the photodiode 50 lies in a second hemisphere quandranton opposite sides of the vane 56. The photodiodes 50 and 52 in each ofthe sensors S2, S1 and S3 form three sets of sensor pairs which will behereinafter referred to as E₁, E₂ ; E₃, E₄ ; E₅, E₆, respectively, wheresensors E₁, E₃ and E₅ lie on the northerly side of the arrays and theremaining sensors lie on the southerly side of the arrays, FIG. 1.

The sensors E₁ -E₄ control the coarse slewing of the CP array. In theevent that the optical array initially is facing in the generaldirection of the sun so that the sensors S1 and S2 receive more lightthan the sensors S3, then control signals derived from S1 and S2 drivethe CP array to within a small angle such as 6° of the final trackingposition as described below. In the event that the optical arrayinitialy is oriented so that its backside 45 (and the position sensorS3) is facing in the general direction of the sun (so that S3 receivesmore light than S1 or S2) then a control signal derived from the S3signal will in a sense override the S1 and S2 signals. In more detail,the S3 signal is employed to produce the motor drive signal for drivemeans 32 and a control signal derived from the gravity sensor S4 isemployed to control the sense in which such drive signal is applied to32, to initially slew the CP array around axis a_(d) to its coarseposition within tracking range. This is discussed later.

The sensors E₁, E₂, E₃, and E₄ may be electrically wired as illustratedin FIG. 7. Sensors E₁ and E₃ serve as two inputs to the non-invertingterminal (+) of amplifier 62 to provide a positive output signal todrive the seasonal drive means 22 in a positive direction about the axisa_(s). Sensors E₂ and E₄ are connected to the inverting (-) inputterminal to amplifier 62 whose output is a negative signal for drivingthe drive means 22 in a negative direction about the axis a_(s).

Thus, when the sensors E₁ and E₃ provide a greater output than thesensors E₂ and E₄, their signal causes the arrays to be rotated in aclockwise direction to decrease the solar energy incident on E₁ and E₃and increase the solar energy incident on E₂ and E₄ due to thestationary effect of the vanes 56 and 60. This provides a seasonaladjustment about the axis a_(s).

Sensors E₁ and E₂ serve also as inputs to the inverting terminal ofamplifier 64. The sensors E₃ and E₄ are connected to the non-invertingterminal of amplifier 64. These sensors provide signals for rotating thearrays about the diurnal axis a_(d). When sensors E₃ and E₄ togetherprovide a greater output than sensors E₁ and E₂, this indicates that thearrays should be rotated in a positive clockwise direction about thepolar axis a_(d). The output of amplifier 64 is applied to the diurnaldrive means 32 via control 66, FIG. 8, to rotate the arrays 12 and 14 inthe positive clockwise direction. When the summed output of the sensorsE₁ and E₂ is equal to the summed outputs of sensors E₃ and E₄ the arraysare aimed at the sun to within some angle such as 6° or so. The sides 42and 46 of the arrays serve as shadow vanes in this mode.

The output of the sensors S1, S2 (and also of sensors S3, S4 and S5) areapplied to control 66, FIG. 8, to be described which provides suitableoutput signals for operating the diurnal drive means 32 or the seasonaldrive means 22 in the proper direction. With respect to sensors S1 andS2, approximately correct aiming about the diurnal axis is achieved whenthe sum of the currents from sensors E₁ and E₂ equals that from sensorsE₃ and E₄. Correct aiming about the seasonal axis is achieved when thesum of the currents from sensors E₁ and E₃ equals that from the sensorsE₂ and E₄. The functions of the remaining sensors S3, S4 and S5 will bediscussed below.

The sensor S4 is secured to the support 24. Sensor S4 is a gravitysensor which provides a signal which indicates the attitude ororientation of the arrays about the diurnal axis. The sensor S4 may be amercury switch as shown in FIG. 4. In FIG. 4, sensor S4 comprises atubular housing 67 into which two electrodes 68 and 70 project at oneend approximately equidistant from the long center line 72 of thehousing. A small amount of mercury 74 inside the housing is free to flowwithin the housing and electrically connects the electrodes 68 and 70when they point toward the earth and electrically open when they pointtoward the sky. When the housing 67 axis 72 is horizontal it is an "Idon't care" condition, as will be explained. The switch conditions ofsensor S4 are designated positive and negative. Sensor S4 is connectedto the control 66 of FIG. 8. When the axis 72 of the switch of FIG. 4 ishorizontal, the axis a_(s) of the arrays of FIG. 1 is also horizontal.This indicates the reference orientation as mentioned above. When theswitch is closed, it indicates the array is at one orientation withrespect to its reference orientation and when open, it indicates thearray is in a second orientation with respect to the referenceorientation.

In FIG. 9, box 76 represents the arrays 12 and 14. 16' indicates theface of the arrays which is to be aimed toward the sun. The sensors S1',S3' and S2' represent the sensors S1, S3 and S2 of FIG. 1. Sensor S3' iscentrally disposed on the bottom of the array 76 of FIG. 9 for purposesof illustration. It will be apparent that this sensor can be displacedin a direction parallel to the seasonal axis a_(s). If we assume thatthe configuration of FIG. 9 is one in which we are looking down at thearray from the sky toward the earth, it is seen that the sky is dividedinto three cylindrical segments formed by planes into the drawing alongdashed lines P₁, P₂ and P₃. The sector formed by the planes of dashedlines P₁ and P₃ encloses space in which the sun causes sensor S1' tohave the greatest output current of the three sensors S1', S2' and S3'.Hereinafter, when the sensors are referred to as having a given output,it is implied that the diode pairs of that sensor are electrically tiedtogether. The planes of dashed lines P₁ and P₂ define the sector inspace in which the sun causes the output of sensor S3' to be a maximum.The planes of lines P₂ and P₃ defines the sector in space in which thesun causes the output of sensor S2' to be a maximum

The planes of the dashed lines define a locus of points of the sun'spositions in which the outputs of two adjacent sensors are equal. Forexample, sensor S1' has an output that is equal to that of sensor S3'when the sun is aimed at the array along the plane of dashed line P₁.The orientation of the sensor S1' is a mirror image of the orientationof the sensor S3' with respect to dashed line P₁. That is, the normal tosensor S1 is 45° with respect to line P₁, while the normal to sensor S3faces in the opposite 45° direction from dashed line P₁. Similarly, theoutputs of sensors S3' and S2' are equal when the sun is aimed at thearray in the plane of dashed line P₂.

If the sun's rays are parallel to the plane in which dashed line P₃ islocated and is aimed at the array, then sensors S1' and S2' will haveequal outputs and indicate that the sun is facing the array which is thecorrect desired orientation. In this orientation, the output of sensorS3 is small and indicates shadow. Lines P₁ and P₂ intersect at 90° andlines P₂ and P₃ at 135° and P₁ and P₃ at 135°.

These relationships can be shown more graphically by mapping the outputsof the various sensors. In FIGS. 10-13, if it is assumed that the CParray is located at a latitute no greater than 60°, then the usefuldaylight illumination would occur when the sun is approximately ±112°from its reference, that is, its local noon position In FIGS. 10-13 theangle θ is the angle of the CP array with respect to its referenceorientation (horizontal) represented by 0°. θ₂ is the angle of the sunwith respect to its reference orientation (at local noon) represented by0°. The graphs of FIGS. 10-13 are plots of the angle θ₁ with respect tothe angle θ₂. θ₁ and θ₂ may each be ±112° about the 0° position. In FIG.10, the line 80 represents the values of θ₁ and θ₂ in which the array isfacing the sun in the correct orientation.

One object of the present system is to avoid the requirement to rotatethe array beyond certain fixed limits defining a certain angle less than360°. This is not a problem with respect to the seasonal axis as thetotal movement possible is only ±231/2°=47°. With respect to the diurnalaxis, however, if the CP array initially is at or close to one extremeof its range, and the sun at or close to the other extreme, the anglebetween the normal to the array and the sun may be greater than 180°. Inthis case, the shortest travel distance for the CP array to reach itsdesired tracking position is through the "opposite" less than 180°sector of a 360° circle and this could involve, after tracking operationresumes, a total travel of the CP array through more than 360°. Thiswould require the use of slip rings whereas it is desired to be able tohard wire the electrical connections to the system.

Keeping the above in mind, the various graphs describe the desiredslewing direction to always insure that the CP array never does rotatebeyond the fixed limits of the predetermined permitted slewing angle. InFIG. 10, the segment of the graph designated "slew clockwise" indicateswhen the array is in the relative orientation indicated by the values ofθ₁ and θ₂, the array should be slewed clockwise to its trackingposition. The slew counterclockwise segment indicates initialorientation of array relative to the sun which requires the array to beslewed counterclockwise to its tracking position. Thus, the line 80represents the locus of values of θ₁ and θ₂ at which the normal to thearray points directly toward the sun in a tracking mode. The control 66,to be described, shown in FIG. 8, applies an output signal to rotate thediurnal axis drive 32 either clockwise (positive) or counterclockwise(negative), in accordance with the sensed position of the array withrespect to the angles θ₁ and θ₂ defined by line 80.

FIG. 11 is a graph of the maximum outputs of sensors S1, S2 and S3 atthe various angles θ₁ and θ₂. The dashed line θ₂ -θ₁ =0 in FIG. 11corresponds to the dashed line P₃ in FIG. 9. This means that when theangle of the sun to its reference orientation is the same as the angleof the normal to the array to its reference orientation, the array isaimed toward the sun. The dashed line θ₂ -θ₁ =135° represents the dashedline P₂ of FIG. 9, at which the outputs of sensors S2 and S3 are equal.The area between 0° and -135° in the map of FIG. 11 represents thesector between dashed lines P₂ and P₃ of FIG. 9. When the sun is in thissector the output of sensor S2 is a maximum. The sector between 0° and+135° represents the sector between dashed lines P₁ and P₃ of FIG. 9.When the sun is in this sector, sensor S1 has an output which is greaterthan the output of the remaining sensors.

When sensor S3 has an output which is greater than the output of theremaining sensors S1 and S2 then its output is employed to control thedrive means 32; however, whether clockwise or counterclockwise, thedrive direction cannot be determined from FIG. 11. Sensor S3 can have amaximum output when the angle of the sun to its local noon referenceorientation has a maximum positive value between points W1 and W4 andthe angle of the array to its reference orientation has a negative valuebetween points W3 and W4. It is also seen that the signal from sensor S3is a maximum when the angle of the array to its reference orientationhas a value between points W3' and W4' and the angle of the sun has avalue between W1' and W4'. Thus there is ambiguity in determining inwhich direction the CP array should be driven.

The attitude or gravity sensor S4 is provided to resolve the ambiguityabove. FIG. 12 shows the condition of the gravity sensor S4, whichindicates the attitude of the array with respect to the array'sreference orientation. That is, sensor S4 indicates when the array isnot horizontal and also the particular direction in which the array istilted. For example, with the array tilted say clockwise in a directionabout the axis a_(d) out of the drawing as shown in FIG. 4, so that theswitch is open, the CP array can be stated to be in its negativeposition. On the other hand, when the array is tilted counterclockwiseabout axis a_(d) relative to its reference horizontal orientation, theswitch closes and the CP array can be stated to be in its positiveposition. When the axis a_(s) is horizontal or almost so, the switch S4may be open or closed; however this does not adversely affect operationbecause in this region the fine tracking signals produced by sensor S5assume control of the system (as will be described).

The graph of FIG. 13 is a superimposition of the maps of FIGS. 10, 11and 12. When the output of sensor S3 is a maximum and the attitudesensor indicates a positive condition, the array must be slewedclockwise. Note that in FIG. 12, "attitude sensor+" is in the sameregion as "sensor 3 maximum" in FIG. 11 which is in the same region as"slew clockwise" in FIG. 10. It can also be seen from FIGS. 10-13 thatwhen sensor S3 has an output that is a maximum and the attitude sensorindicates a negative condition, the array must be slewedcounterclockwise to place it in the correct position. It is seen alsothat when the sensor S1 output is a maximum, the array is slewedcounterclockwise. Similarly, the array is slewed clockwise when thesensor S2 output is a maximum.

The relationship between the desired slewing direction and the outputsof the various sensors may readily be obtained from an examination ofFIG. 13. From FIG. 13 the following table may be derived.

                  TABLE 1                                                         ______________________________________                                        Specification of Correct Slewing Directions                                   Sensor Having Attitude Sensor                                                                           Slew Direction                                      Maximum Output                                                                              Output      About Axis a.sub.d                                  ______________________________________                                        1             +           counterclockwise                                    1             -           counterclockwise                                    2             +           clockwise                                           2             -           clockwise                                           3             +           clockwise                                           3             -           counterclockwise                                    ______________________________________                                    

The embodiment illustrated has the advantage of being tolerant to errorsin the outputs of the various sensors that might arise from partialobscuration of the sensors or electrical imbalance. Specifically, errorsin the outputs of the various sensors that correspond to errors of up to±15° in θ₁ or θ₂ will not change the slewing direction as indicated inTable 1, assuming θ₁ and θ₂ are ±112°. The exception to this occurs asthe relative outputs of sensors S1 and S2 approach the same value whenthe array is nearly aimed at the sun. The errors which occur here arenot important, because there are additional corrections made in responseto control signals provided by the imaging sensors S5 of FIG. 1.

Other embodiments than that described above are possible. For example,the integrating surfaces of the various sensors can be 120° sectors ofspheres instead of hemispheres. The boundaries of the regions in spacewhere various sensors have greatest output in this case would be θ₂ -θ₁=-120°, 0 and 120° instead of -135°, 0° and 135° as illustrated in FIG.9. The correct slewing direction is still shown by Table 1. Thisembodiment is capable of operation in more extreme latitudes (an angularrange of coordinates of FIG. 10 between ±120° instead of ±112°) but isless tolerant of errors. For in this case the θ₂ -θ₁ lines willintersect the ±120° angle of sun lines close to the 0° position.

Still other embodiments combine the angular range of a 120° sector andthe tolerance to error of the hemisphere arrangement, but these are morecomplex than that described above. For example, a second array attitudesensor can be incorporated that indicates whether the orientation of thearray about the diurnal axis is greater or less than about 75° westwardof local noon. If the orientation is greater than 75° westward, theoutput of sensor S3 is multiplied by a factor of approximately 2. Onceagain, the correct slewing direction is as shown in Table 1. Thisembodiment is capable of operation with the angular range of coordinatesin FIG. 10 in excess of ±120°, and with considerable tolerance of error.

Vanes provided the sensors S1 and S2 are not necessary to enhance theirsensitivity. Increased sensitivity does not seem to be particularlynecessary for operation of the system, because with state of the artcomponents, good signal-to-noise ratios can be obtained. What isessential is that the use of vanes can provide considerable immunityfrom the effects of partial obscuration of the integrating sensors. Forexample, consider sensors S1 and S2, as shown in FIGS. 1 and 3, to be 3inch diameter hemispherical integrating sensors and the vanes extend 8inches in front of the hemispheres. When the array is aimed directly atthe sun, the outputs of the sensors S1 and S2 are balanced, i.e., equalto each other. It can be shown that if one of the sensors is 50%obstructed, e.g., by a leaf, the signals from the sensors S1 and S2 willbalance when the array is aimed only about 6° from the sun. The sun inthis case will still be within the field of view of the imaging sensorS5 to be described, so the system will continue to operate unhampered bythe partial obscuration of the integrating sensors. Thus, the vane 56provided sensors S1 and S2 is not for the purpose of increasingsensitivity but for the purpose of reducing the effects of partialobscuration of one or more of the sensors from incident solar radiation.

Because the sensor S3 is located on the bottom of the array and is notsubjected to the partial obscuration problem as much as sensors S1 andS2, a vane for this sensor ordinarily would not be required. To providea balance, however, to the outputs of sensors S1 and S2, a vane 60 isprovided the sensor S3. As the vane 56 of the sensors S1 and S2 shadowsa portion of some of those sensors in the absence of partialobscuration, the output of sensor S3 normally would be greater than thatof S1 for a given angle of the sun, due to the shadowing effect of thevanes 56. Vane 60 balances this effect.

The sensor S5 alluded to above provides a fine tracking signal. Thesensor arrangement including S1-S4 provides coarse tracking signals toan accuracy of, for example, a 6° arc. Since, as mentioned in theintroductory portion, the sun subtends an angle of about 1/2°, this 6°tracking accuracy is insufficient to maintain optimum tracking of thesun. For this reason, the imaging sensor S5 is provided.

The angular field of view of the imaging sensor S5 should be at least 6°to be compatible with the coarse sensor. It is not desirable for theangular field of view to be too large, e.g., greater than 14°. Thislarger field of view may result in appreciable mis-aiming of the arraywhen bright clouds appear adjacent the sun when viewed from the array.In one implementation, a field of view of 9° was found to satisfy theabove restrictions.

In FIG. 6, the sensor S5 includes a rectangular housing 82 which ismounted to the side of array 14. Lens 84 is a convex uncorrected lenswhich is corrected only about the center axis 86. This lens can be maderelatively cheaply so no additional correction segments need be added toit. It is relatively easy to manufacture an inexpensive lens with acorrected central portion. As is known in the optics field, aberrationsin spherical lens occur most frequently at the extremities of the lens.

Sensor S5 includes four tracking photo cells 88 (only two of which arevisible) which are set up to track in the usual manner. One example ofsuch a tracking arrangement is described in more detail in U.S. Pat. No.4,041,307. The focal point for the lens 84, however, is below the cells88 so that the cells are inside of the focal distance. To decrease theprobability that the lens will be completely or almost fully obscured,e.g., by a fallen leaf, the diameter of the lens is fairly large, e.g.,several inches in diameter. A large field of view with a photodetectorof a given size implies that the focal length of the lens is made fairlysmall. This combination of large lens diameter and small focal lengthimplies a wide aperture lens, i.e., one with a low f-number. In thepreferred embodiment an aperture in the neighborhood of f/1 ispreferred.

Immunity from the effects of partial obscuration of a lens is obtainedwhen the detector is at the focus of a well corrected lens. For example,an in focus, unobscured, well corrected lens has a small solid circulararea that represents the focused image. After placing the photodetectorat the focal area of the well corrected lens no problem arises due toobscuration as the unobscured image will remain at the focal point as abright spot. Well corrected lenses, however, with apertures in theneighborhood of f/1 are complex and costly. Since a high degree ofcorrection is required only when the solar illumination is parallel tothe lens axis to insure the on axis image is properly focused, it issufficient to use the lens corrected only for on-axis operation. Thatis, it is only when the sun is on-axis and aiming directly at the arrayin a tracking mode that the focused image of solar illumination shouldbe parallel to the lens axis and the sun's image centered on thedetector.

A simple, aspheric lens is sufficient to meet these constraints. A twoinch focal length, f-1, infinity corrected aspheric Fresnel lensprovides a very high degree of immunity from the effects of partialobscuration.

A simple inexpensive lens with aspherical surfaces can be effectivelyused in the following manner. Such a lens, with high aperture, is notwell corrected, even for on-axis operation. A distant light source witha small angular extent (such as the sun) image which is focused onto thedetector's surface appears as shown in FIG. 14A. The Figure shows afairly sharp, bright central image surrounded by a much largerilluminated region with an indistinct boundary, denoted by the dashedlines. The illumination outside the central image is due largely to theeffects of spherical aberration. The image that results when part of thelens is obscured from one side by an opaque, straight edge object isshown in FIG. 14B. The shape of the central image is not appreciablyeffected but the surrounding illuminated region is eclipsed in anasymmetrical way that would unbalance the signals generated by the fourquadrants of the detector and therefore produce an aiming error.

FIG. 14C shows the appearance of the image when the detector is situatedinside the focus of the unobscured lens. The central image is broadenedwith respect to FIG. 12A. The surrounding illuminated region is slightlysmaller. When the lens in this configuration is obscured as describedabove, the appearance of the images is as shown in FIG. 14D. Theilluminated region surrounding the central image is eclipsed at 92 muchlike it was in FIG. 14B, but now the central image is also eclipsed at90, in the direction opposite from the eclipsing of the surroundingillumination. Furthermore, the degree of eclipsing of the central imagecan be controlled by changing the distance that the detected plane isplaced inside the focal distance. The deleterious effects of lensaberrations on immunity to partial obscuration can therefore becancelled to a larger measure by positioning the detector in a planedifferent from the focal plane of the lens.

For example, with a 1.81 inch diameter, 1.73 inch focal length,symmetrical double convex lens with a detector array 0.32 inches insideof focus, an aiming error of no more than 1/6th degree results, which istolerable in the above system for worst case lens obscurations of up to50%. For different lens configurations the optimum detected placementmay be outside, rather than inside of focus.

The positioning of the sensors S1 and S2 adjacent the back surface ofthe arrays permits the use of the sides 42 and 46 of the arrays 12 and14 of FIG. 1 to also effectively operate as vanes with respect to thesesensors. This effective vane operation also helps to minimizeobscuration effects on these two sensors.

The fine sensor S5 operates over an angular extent slightly greater thanthe angular accuracy of the coarse sensors S1-S4. For example, if thecoarse sensors are accurate to 6° of arc, the sensor 5 operates over anangular extent of about 9° so that its field of view is a conesubtending a 9° angle. Sensor S4 sends a signal to control 66 when thesun is in its field of view. Control 66 then overrides the signals fromsensors S1-S4 to operate the drives 22 and 32. This accounts for the"don't care" state of sensor S4 in this position as mentioned above.Also, the low tolerance for error of the sensors S1-S4 in this 9° rangeis also immaterial due to the overriding effect of sensor S5.

In FIG. 8, the sensors S1-S5 are connected to control 66. Control 66 canbe implemented in many different ways to provide the functions desired.These functions include (1) responding to the drive signals derived fromthe coarse position sensors S1 and S2 to operate the drive means 32 and22 during slewing when the output of S3 is less than that of either S1or S2; (2) responding to the signals from gravity sensor S4 and lightsensor S3 for controlling the coarse slewing of drive means 32 inresponse to a drive signal derived from S3 and in a direction indicatedby S4 when the output of S3 exceeds that of either S1 or S2; and (3)responding to the drive signals derived from the fine position sensor S5to operate the two drive means 22 and 32 during the tracking mode andfor effectively disconnecting the other sensors S1-S4 from the systemduring such tracking mode. By way of example only, following is ageneralized description of one implementation, it being understood thatothers, within the skill of the art, are possible.

Control 66 includes the amplifiers 62 and 64 of FIG. 7 and correspondingamplifiers for the sensor S5. The outputs of amplifiers 62 and 64 are DClevels useful for slewing the CP array to its approximate trackingposition within the field of control of fine tracking sensor S5. Whenwithin tracking range, the output of sensor S5 exceeds a given thresholdand, in one example, pulse width modulated pulses derived from S5 areemployed to drive the drive means 22 and 32 and the outputs derived fromsensors S1 and S2 are disconnected from the drive means. If in theinitial slewing mode the output of S3 exceeds that of S1 or S2, sensorsS3 and S4 cause the control 66 to determine which direction about theaxis a_(d) to slew the CP array in accordance with FIG. 13. In this caselogic elements within 66 control the sense of the application of thedirect current drive signal produced in response to the output of S3 tothe drive means 32 for slewing the CP array in the desired direction anddisconnects the outputs from sensors S1 and S2 from the drive means 32.

The circuit for S3 may include a normally primed gate receptive of theoutput of S3 for applying the output through suitable amplifiers forproducing DC level drive signals only when the S3 output exceeds that ofS1 or S2. Logic within 66 controls the direction of application of theseDC levels to drive means 32 in accordance with the output of gravitysensor S4 for controlling the direction of rotation of the CP arrayduring slewing. This logic also is employed to disconnect the output ofamplifier 64 (FIG. 7) from 32 so that in this mode the S3 derived drivesignals rather than the S1, S2 derived drive signals control the slewingof drive means 32.

When the CP array reaches within 6° of its final position, the finetracking sensor S5 outputs are applied to circuits within control 66which produces the fine drive control signals. In one embodiment thesewere four pulse width modulated signals. These signals are sensed andemployed to produce logic levels which effectively disable the normallyenabled gate at the output of S3 for disconnecting S3 from the systemand disable gates receptive of the drive signals derived from S1 and S2for disconnecting them from the system. Concurrently, the four pulsewidth modulated signals are applied to the drive means 22 and 32 fortaking over fine control thereof for operating these means during thetracking mode. One pulse width modulated signal is applied to the (+)drive input and a second signal to the corresponding (-) drive of one ofthe drive means 22 and the other pair of such signals is applied insimilar fashion to drive means 32. The signal with the greater widthpulses for a drive means determines the direction of rotation of thatdrive means.

As mentioned, the S5 output, when present and of greater than a givenlevel, is employed to remove the S1, S2 and S3 inputs from the drivemeans, for example, by disabling gates through which the amplifiedsignals of these sensors flow. In this way sensor S5 overrides theoutputs of sensors S1, S2, S3 and S4. When the sensor S5 output is low,meaning the system is in the coarse tracking mode outside the field ofview of sensor S5, the output (if any) of sensor S5 has essentially noeffect on the system operation.

What is claimed is:
 1. In a solar tracking system including solar sensormeans and control means arranged to track the sun in a diurnal cycle,said sensor means including spaced photovoltaic generator means andlight diffusing means over said generator means wherein the improvementtherewith comprises:vane means adjacent each said generator meansoriented and dimensioned to reduce the sensitivity of said generatormeans to the shadowing effect of partial obscuration of said lightdiffusing means by foreign matter, said diffusing means including ahemispherical light integrating means having a given diameter, said vanemeans extending in front of the hemisphere toward the sun a distance ofabout 2.5 times said diameter.
 2. A solar tracking systemcomprising:first solar sensor means for providing a first solar trackingsignal within a given range from the locus of points defined by theangle of the sensor means to a first reference orientation of the systemand the angle of the sun to a second reference orientation of the sun atwhich locus the sensor means are aimed at the sun, said sensor meansincluding an integrating hemisphere having a given diametricaldimension, second sensor means for providing a second solar trackingsignal within a given range from the locus of points defined by theangle of the second sensor means to said first reference orientation andthe angle of the sun to said second reference orientation at which locusthe second set of sensors are aimed at the sun, said first sensor meansnormally facing the sun during tracking, said second sensor meansincluding sensor means normally facing away from the sun and are inshadow during said tracking, control means responsive to said signalsfor orienting said sensor means in a tracking mode, and vane meansadjacent said first sensor means for reducing the sensitivity of saidfirst sensor means to partial obscuration by foreign matter, said vanemeans extending from said sensor means in a direction toward said sun adistance of about 2.5 times said given diametrical dimension.